Protective redundant subsystem for power tools

ABSTRACT

A protective redundancy circuit is provided for a power tool having an electric motor. The protective redundant subsystem is comprised of: a motor switch coupled in series with the motor; a motor control module that controls the switching operation of the motor switch; and a protective control module that monitors switching operation of the motor switch and disables the power tool when the switching operation of the motor switch fails. In the context of an AC powered tool, the switching operation of the motor switch is correlated to and synchronized to the waveform of the AC input signal. During each cycle or half cycle, the motor control module introduces a delay period before closing the motor switch and the protective control module determines the operational status of the motor switch by measuring the voltage across the motor switch during the delay period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/239,959, filed on Sep. 4, 2009. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to power tools and, more particularly, toa protective redundant subsystem for power tools.

BACKGROUND

Phase control is one commonly employed method for controlling thevoltage applied to a motor in a power tool. Motor operation iscontrolled by switching the motor current on and off at periodicintervals that are synchronized with the alternating current (AC) inputsignal. The switching operation is achieved through the use of anelectronic switch, such as a triac, coupled in series with the motor.One potential concern for phase-controlled power tools is that the triaccan fail. Failure of the triac can be melting, fusing, or cessation ofcommutating off, either permanently or temporarily. This concern isrelatively small for power tools having mechanical power switches thatenable the tool operator to turn off the motor even if the triac were tofail. However, this concern poses a greater concern as toolmanufacturers look to replace the power switches with switches that donot conduct the power being delivered to the motor.

Many power tools also implement a “no-volt release” feature. Briefly, ano-volt release feature prevents a power tool from operating when it isplugged into an AC power outlet when the power switch is in an ONposition (i.e., closed). In a typical implementation, the no-voltrelease feature will not prevent the tool from operating if the triacshort circuits, thereby causing inadvertent tool operation. Therefore,it is desirable to provide a protective redundant subsystem thatmonitors the switching operation of a triac in a power tool application.

This section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

A protective redundant system is provided for a power tool having anelectric motor. The protective redundant system is comprised of: a motorswitch coupled in series with the motor; a motor control module thatcontrols the switching operation of the motor switch; and a protectivecontrol module that monitors switching operation of the motor switch anddisables the power tool when the switching operation of the motor switchfails. In the context of an AC powered tool, the switching operation ofthe motor switch is correlated to and synchronized to the waveform ofthe AC input signal. During each cycle or half cycle, the motor controlmodule introduces a delay period before closing the motor switch and theprotective control module determines the operational status of the motorswitch by measuring the voltage across the motor switch during the delayperiod.

In one aspect of the subsystem, the protective control module measuresvoltage across the motor switch when the phase of the AC input signal isless than a predetermined phase.

In another aspect of the protective redundancy circuit, a protectiveswitch is connected across an AC line carrying the AC input signal and afuse is disposed in the AC line, where the protective control modulecloses the switch when the measured voltage indicates a malfunction,thereby shorting the AC line and opening the fuse. Alternatively, theprotective switch may be connected across the motor, where theprotective control module closes the switch when the measured voltageindicates a malfunction, thereby shorting the AC line and opening thefuse.

The protective redundant system may further comprise a power on/offswitch disposed between an AC power source and the motor, wherein theprotective control module is operational only when the power on/offswitch is in a closed position. Alternatively, the power on/off switchis disposed between an AC power source and the motor, wherein theprotective control module receives an input indicative of position ofthe power on/off switch and operates in accordance with the input. Inanother instance, the power on/off switch is interfaced with the motorcontrol module but does not conduct the power delivered to the motortherethrough, wherein the protective control module receives an inputindicative of position of the power on/off switch and operates inaccordance with the input.

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

FIG. 1 is a simplified circuit diagram for controlling a motor of apower tool;

FIG. 2 illustrates the voltage across a phase-controlled switch inrelation to the AC supply signal;

FIG. 3 is a flowchart illustrating the operation of an exemplaryprotective control module;

FIG. 4 is a circuit diagram of another exemplary embodiment of theprotective control module;

FIG. 5 is a circuit diagram of an alternative circuit arrangement forthe motor control system;

FIG. 6 is a circuit diagram of a variant of the protective controlmodule which accounts for the state of the power on/off switch;

FIGS. 7A and 7B are circuit diagrams that illustrate another variant ofthe motor control system and the protective control module,respectively, which accounts for the state of the power on/off switch;

FIG. 8 is a circuit diagram for a motor control system having aprotective switch across the motor;

FIG. 9 is a circuit diagram for a motor control system having a poweron/off switch that does not conduct the power delivered to the motor;

FIG. 10 is a circuit diagram of another variant of the protectivecontrol module; and

FIG. 11 is a diagram depicting an alternative embodiment of theprotective redundant subsystem.

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure. Correspondingreference numerals indicate corresponding parts throughout the severalviews of the drawings.

DETAILED DESCRIPTION

FIG. 1 depicts a simplified circuit diagram for controlling a motor in apower tool. The motor control system 10 is comprised generally of apower on/off switch 12, an electric motor 14, a motor switch 16 coupledin series with the motor 14, and a motor control module 18 interfacedwith the motor switch 16. In an exemplary embodiment, the motor is auniversal motor and the motor switch is a triac but it is readilyunderstood that other types of motors and switches fall within the scopeof this disclosure. For example, the motor may be an AC motor or DCmotor, including brushed or brushless; whereas, the motor switch may bea field effect transistor (FET), an insulated gate bipolar transistor(IGBT), a silicon-controlled rectifier (SCR), or another type ofelectronic switch. The motor switch may also be replaced with anH-bridge, half-bridge or some other motor switching arrangement. In someembodiments, the motor switch may be incorporated into the motor controlmodule. The motor control system is connectable to a power source. Inthe exemplary embodiment, the motor control system is connectable to anAC power source at AC main node and AC neutral node. The connection maybe made in a conventional manner via a power cord to an AC outlet.However, it is envisioned that the protective redundant subsystemfurther described below is applicable to power tools having DC powersources as well.

During operation, the motor control module 18 controls the amount ofvoltage applied to the motor 14 by controlling the switching operationof the motor switch 16. As used herein, the term module may refer to, bepart of, or include an electronic circuit, an Application SpecificIntegrated Circuit (ASIC), a processor (shared, dedicated, or group)and/or memory (shared, dedicated, or group) that execute one or moresoftware or firmware programs, a combinational logic circuit, and/orother suitable components that provide the described functionality.

In an exemplary embodiment, the motor control module 18 employs phasecontrol to control the amount of voltage applied to the motor 14.Generally, operation of the motor 14 is controlled by switching themotor current on and off at periodic intervals in relation to the zerocrossing of the AC input signal. These periodic intervals are caused tooccur in synchronism with the waveform of the AC signal and are measuredin terms of a conduction angle, measured as a number of degrees. Theconduction angle determines the point within the AC waveform at whichthe motor switch is fired (i.e., closed), thereby delivering current tothe motor. For example, a conduction angle of 180° per half cyclecorresponds to a condition of full or maximum conduction. That is, themotor switch 16 is fired such that current flows through the switch forthe entire half cycle of the AC input signal. Similarly, a 90°conduction angle commences current flow in the middle of the half cycleand thus half the available energy is delivered to the motor. Thus, theconduction angle determines the point at which the motor switch isfired. While the following description is provided with reference tophase control, it is readily understood that other motor control schemes(e.g., pulse width modulation) are within the broader aspects of thisdisclosure.

The motor control system 10 further includes a protective redundantsubsystem 20. The protective redundant subsystem 20 monitors theswitching operation of the motor switch 16 and initiates one or moreprotective operations to protect the tool operator when the switchingoperation of the motor switch fails. Different arrangements for theprotective redundant subsystem are contemplated as will be furtherdescribed below.

With continued reference to FIG. 1, the protective redundant subsystem20 may be comprised of a protective control module 22, a protectiveswitch 24, and a circuit breaker 26 and/or fuse 27. In this exemplaryembodiment, the protective control module 22 is configured to measurethe voltage across the motor switch, as will be described later below.

In power tools with universal motors, current typically lags behind thevoltage by a few degrees. This delay affects the operation of the triac.Specifically, the triac turns on by the motor control module and turnsoff at the current zero cross. Thus, it may be important not to turn thetriac on too early in an AC voltage half cycle before it has had theopportunity to commutate off at the current zero cross ending the lastcurrent half cycle. Turning on the triac too early might result in afull half cycle of non-conduction, unless there is a provision for oneor more re-firings of the triac within that half-cycle.

In order to ensure that the triac is not turned on too early, some delayis introduced between the AC voltage zero cross and the firing of thetriac. The net effect is some delay from the true AC current zero crossuntil the triac can be fired in a maximum conduction situation. Thissmall amount of electrical delay is introduced by the motor controlmodule 18 and does not cause any major effect in the operation of thepower tool (e.g., less than 0.01% RMS voltage is dropped across thetriac).

During this controlled delay, an instantaneous voltage develops acrossthe triac following every voltage zero crossing. As shown in FIG. 2, thevoltage spike 28 is developed across a phase-controlled switch inrelation to the AC supply signal and has a saw-tooth shape.Specifically, the voltage spike appears immediately following thecurrent zero crossing (not shown in FIG. 2) and ends when the triac isturned on. For example, on an exemplary 90 volt RMS AC supply, aninstantaneous voltage of almost 11 volts will be developed across thetriac before the device is turned on during normal phase-controloperation. During maximum conduction, the motor control module 18 closesthe triac after a short delay period (e.g., 5 degrees) from the voltagezero-crossing of the AC input signal as indicated at 29. Thus, theswitching operation of the triac can be monitored by measuring thevoltage across the triac during, for example, the first 5 degrees ofnon-conduction of every AC cycle (or half-cycle). It is understood thatthere is no need for the delay period when the prescribed conductionangle is less than maximum, i.e., when the firing point or closing ofthe triac exceeds the delay period (the firing point being thecomplement of the conduction angle such that a conduction angle of 135degrees is a firing point of 45 degrees and a conduction of 60 degreesis a firing point of 120 degrees). In other words, the motor controlmodule 18 may introduce a delay during every AC cycle (i.e., firingpoint angle+delay period angle) or only when the prescribed firing pointis less than the delay period. Nonetheless, the protective controlmodule 22 measures voltage across the triac preferably during the delayperiod (i.e., when the phase of the AC input signal is less than a phasesignifying the delay period) but in any case during a non-conductionperiod of every AC cycle before the triac is closed.

In an exemplary implementation, the protective control module 22 mayfunction in the manner shown in FIG. 3. The protective control modulewill continually monitor the voltage across the triac as indicated at31. The voltage is expected to develop across the switch during, forexample, the first 4 or 5 degrees of non-conduction of every AC cycle.When the measured voltage exceeds some predetermined threshold (e.g., 7volts), then the triac is operating properly. In this case, a timer isreset at 33 and voltage monitoring continues. However, when the measuredvoltage does not exceed the threshold during some expected timeinterval, then the triac is presumed to have failed (e.g., fused in aclosed position) and the protective switch is closed at 36. The expectedtime interval and thus the timer duration are correlated to the lengthof the AC cycle and may be one AC half cycle, one full AC cycle, or anymultiple of AC half cycles. In an exemplary embodiment, the abovefunctionality may be implemented by software instructions embedded in amicroprocessor. It is to be understood that only the relevant steps ofthe methodology are discussed in relation to FIG. 3, but that othersoftware instructions, or other hardware, may be needed to control andmanage the overall operation of the system. The above functionality mayalso be implemented by hard-wired electronic circuits as furtherdescribed below.

When the measured voltage indicates the triac has failed, the protectivecontrol module 22 will initiate some protective operation to protect theuser. Exemplary protective operations may include (but are not limitedto) tripping a circuit breaker, blowing a fuse (including positivetemperature coefficient resettable fuses), disabling the motor controlscheme (e.g., phase control), disconnecting power to the motor (e.g., byopening a switch) or other otherwise disabling the operation of thepower tool.

Referring back to FIG. 1, a protective switch 24 is connected across theAC line (i.e., coupled between the AC main line and the AC neutral orreturn line). In addition, a circuit breaker 26 and/or a fuse 27 isdisposed in the circuit path between the AC power source and theprotective switch. In this embodiment, the protective control module 22closes the protective switch 24 when the triac 16 fails, therebyshorting the AC line. The sudden flow of current will trip the circuitbreaker and/or blow the fuse, thereby disabling operation of the powertool. Protective switch may be implemented using a field effecttransistor (FET), an insulated gate bipolar transistor (IGBT), asilicon-controlled rectifier (SCR), a triac, a solid-state relay, amechanical relay or another type of switch.

It is contemplated that there may be insufficient energy to blow thefuse or other protective device. An alternative embodiment for theprotective redundant subsystem 20 is shown in FIG. 11. In thisembodiment, a capacitor 112 is placed in parallel with the protectiveswitch 24. During normal operation of the motor 14, the capacitor 112 ischarged through a diode 114 and a resistor 116 that limits the inrush ofcurrent into the capacitor. When the protective control module 22detects the triac has failed, it closes the protective switch 24. Theenergy stored in the capacitor 112 discharges into the fuse and therebyblows the fuse. It is envisioned that the capacitor 112 may be replacedwith or used in combination with other types of energy storing devices,such as inductors. It is further understood that the protectiveredundant subsystem 20 may be reconfigured to support of such devices.

When the triac 16 fails, the operator cannot resume operation by merelyreplacing the fuse or resetting the circuit breaker. Therefore, it iscontemplated that the protective redundant subsystem (or at least aportion thereof including the blown fuse and the failed motor switch)may be configured as a replaceable cartridge that can be replaced by theoperator to resume tool operation. Other techniques for disabling thetool as well as other types of protective operations are contemplated bythis disclosure.

In an alternative implementation, a temperature sensor may be used todetermine the operational status of the triac. The temperature sensormay be placed proximate to the triac. When the temperature measureexceeds some threshold, it may likewise be presumed the triac has failedor is likely to fail. It is envisioned that other measures orcombination of measures, like voltage and/or current, may be used todetermine the operational status of the triac.

FIG. 4 illustrates another exemplary embodiment of the protectivecontrol module 22. The protective control module 22 is comprised of adifferential amplifier 41, a comparator 42, a transistor 43 and acapacitor 44. The differential amplifier 41 may be an operationalamplifier; whereas, the comparator 42 may be an operational amplifier oran integrated circuit comparator.

In operation, the differential amplifier 41 senses the voltage acrossthe motor switch (not shown). There will be some measureable voltageacross the switch so long as the switch is not fused. The differentialamplifier 41 acts to attenuate and low-pass filter the measured voltage,thereby discriminating against high frequency noise. Output from thedifferential amplifier 41 is in turn compared by the comparator 42 to apredefined voltage threshold. When the output from the differentialamplifier 41 exceeds the voltage threshold, the comparator 42 outputs asignal to the transistor 43. The input signal to the transistor 43causes the transistor to conduct, thereby shorting any charge that hasaccumulated on the capacitor to ground. A DC power supply 46 iscontinually charging the capacitor 44 through the resistor 45. Shouldthe transistor 43 fail to short the charge on the capacitor 44, then thevoltage across the capacitor will build to the point where the gatecircuitry of the protective switch 24 (e.g., a triac; not all gate drivecircuitry shown) fires and thereby shorts the AC line. This will occurwhen there is no measureable voltage across the switch 16 and thuscomparator 42 does not output a signal to the transistor 43. Inpractice, it may be necessary to connect a monostable multivibrator tothe output of the comparator in order to lengthen the time of shortpulses which cause the transistor to conduct. The monostablemultivibrator may be a negative recovery monostable multivibrator thatis retriggerable. Furthermore, a latching circuit may be needed toensure that once the triac fires it is not extinguished by subsequent ACphase reversals; an exemplary implementation of the multivibrator andthe latching circuit are shown in FIG. 6. Furthermore, this latchingcircuit may repeatedly fire the protective switch 24 every AC half-cycleuntil the fuse opens or the circuit breaker blows. Such latchingcircuitry is shown in FIG. 10 as a zero cross detection circuit and agated monostable multivibrator. It is noted that this embodiment doesnot protect against faults in the power on/off switch.

FIG. 5 illustrates an alternative circuit arrangement for the motorcontrol system. In this arrangement, the protective control module 22can sense the position of the power on/off switch 12. To do so, thepower on/off switch 12 may be implemented as a double pole switch havinga second set of contacts which are used by the protective control module22. Other techniques for determining the position of the power on/offswitch are also envisioned.

FIG. 6 illustrates a variant of the protective control module 22 whichaccounts for the state of the power on/off switch and may be used in thearrangement shown in FIG. 5. When the power on/off switch 12 is in theon position (i.e., closed) and the motor switch 16 is fused, then theprotective circuit 20 functions to short the AC line. When the poweron/off switch 12 is in the off position (i.e., open), then there is nopower through the motor 14 and thus no voltage across the motor switch16. In the embodiment illustrated in FIG. 4, when there is no voltageacross the motor switch 16, the protective circuit 20 would function toshort the AC line. In the variant of the protective control module 22illustrated in FIG. 6, when the power on/off switch is off, thealternate contact shown at 61 is closed. As a result, the capacitor 44is shorted to ground and the AC line cannot be shorted while the poweron/off switch is off.

FIG. 7A illustrates another circuit arrangement for the motor controlsystem which accounts for the state of the power on/off switch. Theprotective control module 22 illustrated in FIG. 7B further includes asecond differential amplifier 71 and a second comparator 72 in place ofthe second pole of the double pole power on/off switch depicted in FIG.5.

FIG. 7B illustrates a variant of the protective control module which maybe used in the arrangement shown in FIG. 7A. The protective controlmodule 22 in FIG. 7B is similar to the one depicted in FIG. 4 withsimilar components having the same reference numbers. With reference toFIG. 7B, there are two circuit paths which provide input to transistor43. The protective switch 24 is closed only when the output from bothpaths is a logic low signal. On the other hand, if the output fromeither path is a high voltage signal, then the capacitor 44 is shortedto ground and the protective switch 24 remains open. For instance, whenthe power on/off switch 12 is in an ON position (i.e., closed), there isno voltage across the power on/off switch 12 and the output from theupper circuit path is a logic low signal. The output from the lowercircuit path follows the explanation provided in reference to FIG. 4.That is, there will be some measureable voltage across the motor switch16 so long as the motor switch is not fused. This signal willperiodically cause the transistor to conduct, thereby shorting thecapacitor to ground and keeping the protective switch 24 open. Shouldthe motor switch short circuit, the output from the lower circuit pathremains a logic low signal. As a result, there is no input signal to thetransistor 43 and the subsequent build up of voltage across thecapacitor will close the protective switch 24. When the power on/offswitch 12 is in an OFF position (i.e., open), the output from the uppercircuit path is a logic high signal. In this scenario, the capacitor isshorted to ground and the protective switch 24 remains open when thetool is not operational.

FIG. 8 illustrates another alternative circuit arrangement for the motorcontrol system. In this arrangement, the protective switch 24 isconnected across the motor. This arrangement alleviates the need forsensing the state of the power on/off switch. When the power on/offswitch is off (i.e., open position), there is no voltage across themotor switch so that the protective control module closes the protectiveswitch. Any current shorted around the motor is not sufficient to tripthe breaker or blow the fuse when the power on/off switch is in an offposition. Conversely, when the power on/off switch is on (i.e., closedposition), the protective control module 22 operates in the mannerdescribed above in relation to FIG. 1.

Preceding embodiments contemplate a power on/off switch that is disposedbetween the AC power source and the motor such that the power deliveredto the motor is conducted through the power on/off switch. FIG. 9illustrates an alternative embodiment of a protective redundantsubsystem that is suitable for use with a power on/off switch that doesnot conduct the power delivered to the motor. In this exemplaryembodiment, the power on/off switch 91 is interfaced with the motorcontrol module 18 and provides an indicator thereto as to whether a userdesires the tool to be operational or not. The motor control module 18may in turn control the amount of voltage applied to the motor 14 in themanner described above. The power on/off switch 91 may be implemented asa push button, a sliding switch or some other type of “logic” switch.Rather than being connected across the AC line, it is also envisionedthat the protective switch 24 may be connected across the motor 14.

The protective control module 22 can also sense the state or position ofthe power on/off switch 91. To do so, the power on/off switch 91 may beimplemented as a double pole switch having a second set of contactswhich are used by the protective control module 22. In this way, theprotective control module 22 receives an input indicative of position ofthe power on/off switch 91 and operates in accordance with the input.During operation, the protective control module 22 will function toshort the AC line in the manner described above when the power on/offswitch 91 is in the on position (i.e., closed) and the motor switch 16is short circuited. Likewise, the protective control module 22 isconfigured to short the AC line when the power on/off switch 91 is inthe off position (i.e., open) but the motor switch 14 is fired by themotor control module 18. In this way, the protective control module 22also serves to protect against a fault in the motor control module 18.This additional protective measure is particularly important since thepower on/off switch 91 does not conduct the power delivered to themotor.

In an alternative embodiment of the protective control module 22 shownin FIG. 9, the protective control module 22 may disable monitoring ofthe motor switch when the power on/off switch is in the off position(i.e. open) and thus the tool is not operational. For this exemplaryembodiment, the protective control module 22 may be implemented as shownin FIG. 6. Other techniques for determining the position of the poweron/off switch as well as other implementations for the protectivecontrol module 22 are also envisioned by this disclosure.

Power tools incorporating the protective redundant subsystem describedabove may also be configured with a “no-volt release” function. If thepower on/off switch is in the On position and the AC power is applied tothe tool, the “no-volt release” function will prevent the tool fromoperating. Only upon opening the power on/off switch to the Off positionand subsequently closing the on/off switch to the On position will thetool become operational. Thus, the “no-volt release” function maymonitor the position of the power on/off switch. Alternatively, the“no-volt release” function may monitor power through the motor switch toinfer whether the power on/off switch has been actuated to the Offposition. Further details regarding an exemplary “no-volt release”function may be found in U.S. Pat. No. 7,551,411, which is incorporatedherein by reference in its entirety.

In the event that the motor switch becomes shorted, the “no-voltrelease” function may not operate properly, especially when the powertool is configured with a power on/off switch that does not conduct thepower being delivered to the motor. In this situation, the protectiveredundant subsystem works cooperatively with the “no-volt release”function to prevent unsafe operation of the tool. The “no-volt release”function is typically implemented by the motor control module. Howeverthe protective redundant subsystem and the “no-volt release” functionmay be combined into a single module. The protective redundant subsystemand the motor control subsystem may also be combined into one module.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention. Exampleembodiments are provided so that this disclosure will be thorough, andwill fully convey the scope to those who are skilled in the art.Numerous specific details are set forth such as examples of specificcomponents, devices, and methods, to provide a thorough understanding ofembodiments of the present disclosure. It will be apparent to thoseskilled in the art that specific details need not be employed, thatexample embodiments may be embodied in many different forms and thatneither should be construed to limit the scope of the disclosure. Insome example embodiments, well-known processes, well-known devicestructures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a”, “an” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

What is claimed is:
 1. A power tool configured to receive an alternatingcurrent (AC) input signal, comprising: an electric motor; aphase-controlled switch coupled in series with the motor; a motorcontrol module interfaced with the phase-controlled switch thatmodulates the conductive state of the phase-controlled switch inrelation to a zero-crossing of the AC input signal; and a protectivecontrol module that measures voltage across the phase-controlled switchduring a non-conductive state of the phase-controlled switch after azero-crossing of the AC input signal but prior to closing of thephase-controlled switch and disables the power tool when the measuredvoltage indicates a malfunction of the phase-controlled switch.
 2. Thepower tool of claim 1 wherein protective control module measures voltageacross the phase-controlled switch when a phase of the AC input signalis less than five degrees.
 3. The power tool of claim 1 wherein theprotective control module sustains operation of the power tool whenvoltage spikes exceeding a predetermined voltage are detected across thephase-controlled switch and disables the power tool upon failing todetect the voltage spikes across the phase-controlled switch.
 4. Thepower tool of claim 1 further comprises a protective switch connectedacross an AC line carrying the AC input signal and a fuse disposed inthe AC line, where the protective control module closes the switch whenthe measured voltage indicates a malfunction, thereby shorting the ACline and opening the fuse.
 5. The power tool of claim 1 furthercomprises a protective switch connected across the motor and a fusedisposed in the AC line, where the protective control module closes theswitch when the measured voltage indicates a malfunction, therebyshorting the AC line and opening the fuse.
 6. The power tool of claim 1further comprises a fuse disposed in the AC line, an energy storingdevice, and a protective switch in series with the energy storingdevice, wherein the energy storing device and the protective switch arearranged in parallel with the fuse to short the AC line and open thefuse when the measured voltage indicates a malfunction.
 7. The powertool of claim 1 further comprises a power on/off switch disposed betweenan AC power source and the motor, wherein the protective control moduleis operational only when the power on/off switch is in a closedposition.
 8. The power tool of claim 1 further comprises a power on/offswitch disposed between an AC power source and the motor, wherein theprotective control module receives an input indicative of position ofthe power on/off switch and operates in accordance with the input. 9.The power tool of claim 1 further comprises a power on/off switchinterfaced with the motor control module but does not conduct the powerdelivered to the motor therethrough, wherein the protective controlmodule receives an input indicative of position of the power on/offswitch and operates in accordance with the input.